802.1Q

Justin Ellis3:53 pm

The open IEEE standard 802.1Q supports quality of service (QoS) and Virtual LAN (VLAN) identification as traffic moves over an Ethernet network. A VLAN is a local area network that utilizes another LAN, rather than its own physical network, to carry data. The 802.1Q tagging protocol allows the Ethernet frame size to increase by four bytes to a range of 68 to 1522 bytes. This size increase is due to the insertion of a four-byte VLAN tag into the frame. The tags, which include a VLAN Identifier (VID), are attached to each Ethernet frame by MAC address. This 12-bit VID is assigned to every VLAN, with 4094 identifiers available for use.

802.1Q VLAN Traffic

Switches that are compatible can carry 802.1Q VLAN traffic. Devices can be connected to these switches through a trunk port or access port. When a trunk port is used, the connected device receives tagged Ethernet frames. If an access port is the connection point, a device sees untagged frames. The 802.1Q standard supports the unique configuration of individual VLANS. Network equipment providers offer 802.1Q-compatible devices.

QoS Prioritization with 802.1Q Standard

The QoS prioritization with the 802.1Q standard is supported by the 802.1p technique. With this protocol, three bits are used to assign one of eight Priority Code Points (PCPs) to traffic. The PCP prioritizes classes of traffic, including video, voice or data. IEEE provides some recommendations regarding the priority level to utilize with specific traffic formats. For example, Network Control has a recommended PCP of 7.